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This study by Gregory Antonios and the group of Thomas Bayer—and their previous study in 2013—pertains to actions of their antibody, NT4X, in a broad array of assays and models in vitro, ex vivo, and in vivo.

The data mainly confirm the seminal work of Schenk et al., 15 years ago: preclinical validation of immunotherapy as potential cure of AD, and by extension other neurological diseases that have neurotoxic protein-aggregates as cause or correlate—notwithstanding the fact that we fail to understand the underlying molecular mechanisms.

When immunotherapy proves effective—and it does so in mice—the medical benefits might even be overshadowed by what we learn about neuroscience. I advocated on many an occasion that when immunotherapy does make it into the clinic, as everybody hopes, it will help us understand some of the most fundamental biochemical and cellular processes in our brain—something you would normally try to do before developing a treatment. Among the many mysteries that are still unsolved, despite being chased for decades by so many great minds, are: the origin and spreading of pathology, the identity of neurotoxic protein species, the physiological function (if any) of amyloid peptides, network-functional interrelations of brain regions, the ApoE conundrum, immune surveillance, the role of the blood brain barrier, and many more …

At face value, and given our own efforts in this sub-field, one is no longer surprised that immunotherapy works in mice, although with respect to the actual models used here, several caveats are in order:

Despite their convincing reports and data, provided here and previously, it is still early days for the hypothesis that the truncated Aβ4-4X, the Tg4-42 mice, and the NTX4 antibody are important in/for AD patients—they all need wider and deeper studies, not just in mice but also in human neurons and brains.

The NT4X antibody was tested here mainly in “purpose-built” mice that express truncated Aβ4-42 from a Thy1-TRH gene promoter/processing construct, obviating the complex routes of cellular trafficking and proteolyting processing of the parent APP. Tg4-42 mice show cell-associated amyloid pathology at very young ages, with severe hippocampal neuron death from age five to eight months onwards, translating not surprisingly into severe defects in learning and memory (Bouter et al., 2013).

Model and data stemming from them that depends on intracerebroventricular injection of, in this study Aβ4-42, but in general of any synthetic peptide, must be taken with caution.

The inclusion or add-on of the 5XFAD model is, compared to the Tg4-42 mice, rather brief in terms of assays and data, while the authors go to great lengths explaining the less-than-impressive outcome. This reminded us of the genetically complex construction of these mice that can model all but sporadic AD.

Lastly, a rather strange counterpoint surfaced of the current study in comparison with their first NT4X report (Antonios et al., 2013), which stated in the abstract: “While NT4X-167 significantly rescued Aβ4-42 toxicity in vitro no beneficial effect was observed against Aβ1-42 or AβpE3-42 toxicity”—as opposed to the current claim that the antibody acted effectively not only against the 4-42 but also against pE3-42 amyloid peptides. This raises interesting questions concerning the methodology used in both studies, and even more so about the actual epitope that is defined or recognized by the NT4X Mab, which they claimed to be totally dependent on the F4 residue.

As usual, interesting aspects addressed by fine research generate even more interesting questions …

What evidence is there for N-truncated pyroglutamate Aβ3-42 and Aβ4-42 as therapeutic targets of Alzheimer’s disease?

As early as 1985, truncated Aβ peptides were described as precipitating in AD plaques, including a major species beginning with phenylalanine at position 4 ( Aβ4-x) (Masters et al., 1985). A majority of 64 percent of the peptides in amyloid plaques of two sporadic AD cases and 45 percent of the peptides in the patients with Down’s syndrome started with a Phe-4 residue. At the same time, Glenner and Wong demonstrated that full-length Aβ beginning with Asp-1 was the main species detected in cerebrovascular deposits (Glenner and Wong, 1984). There is common agreement that plaque-associated Aβ peptides mainly terminate at Ala-42. Analyzing familial AD patients, Ancolio and colleagues (Ancolio et al., 1999) first showed a selective and drastic increase of N-truncated Aβx–42 species triggered by the mutation APP V715M, postulating that all Aβx–42 variants are main factors driving AD pathology. Miller et al. compared the peptide compositions of the cerebrovascular and senile plaque core amyloid deposits in AD. Matrix-assisted, laser-desorption-time-of-flight (MALDI-TOF) mass spectrometry of plaque-Aβ revealed an array of peptides ending with Ala-42, while cerebrovascular Aβ began with Arg-1 ending at Val-40. They verified that Phe-4 is the main component in plaques, but cautioned that their MALDI-TOF spectral data suggest the presence of two pyroglutamyl amino termini (pyroGlu-3 and pyroGlu-11) that might escape detection by other methods. Using immunoprecipitation in combination with mass spectrometric analysis, the dominating Aβ isoforms in the three different brain regions analyzed from control, sporadic, and familial AD were described as Aβ1-42, AβpE3-42, Aβ4-42, and Aβ1-40, with Aβ1-42 and Aβ4-42 being the dominant isoforms in the hippocampi and cortices in all groups analyzed (Portelius et al., 2010). The question whether N-truncations of Aβ are a postmortem artifact or might even precede the symptomatology of AD was addressed by Sergeant and co-workers (Sergeant et al., 2003). They adapted a proteomic method in combination with western blotting and mass spectrometry for the characterization of insoluble Aβ extracted in formic acid. Full-length Aβ peptides represented 37 percent of all Aβ species, while 17 percent corresponded to N-truncated species starting at residues Phe-4 and Arg-5, and 20 percent started with Ser-8, Gly-9, and Tyr-10. They also demonstrated that at the first stage of amyloid deposition in non-demented individuals, plaques comprised N-terminal truncated variants starting at positions 4-, 5-, 8- and 9–42, or with a pyroglutamyl residue at position 3. At this stage, Aβ oligomers were exclusively made of Aβx-42 species.

In a recent review (Bayer and Wirths, 2014), we summarized these arguments and discussed the validity of available mouse models focusing on Aβ3-42 and Aβ4-42 as Alzheimer’s targets.

N-truncated Aβ variants, predominantly with pyroGlu-3 and Phe-4, correlate well with presymptomatic AD.

N-truncated variants pyroGlu-3 and Phe-4 Aβ peptides are more abundant than full-length Aβ in the brains of patients diagnosed with sporadic or familial AD.

The comments by Fred van Leuven are well accepted. Of course, the Tg4-42 animal model was designed to test Aβ4-42-induced pathology (Antonios et al., 2015). We wanted to ask whether full-length and Fab fragement of NT4X administered peripherally can act in the central nervous system, even reaching aggregates within neurons. The neuron loss is fast in Tg4-42, reaching 50 percent at six months of age. It was significantly slowed down after 12-week-long NT4X treatment. As a consequence, the mice performed better in the memory test with both the full-length and Fab antibodies. Seemingly, there is agreement that NT4X can act in the CNS, even protecting against neuronal degeneration, a therapeutic feature not reported by other antibody treatments (so far). Moreover, we think that the therapeutic mechanism does not involve the Fc-part of the antibody (i.e., microglia clearence), as the Fab worked as well as the full antibody. As a matter of fact, NT4X treatment of 5XFAD mice lowered the plaque load of N-truncated peptides, but not of full-length Aβ1-42, as expected, because the 5XFAD model produces only low levels of AβpE3-42 and Aβ4-42. This raises a question about the face validity of animal models for human disorders in general.

We completely agree that wider and deeper studies ultimately will be needed in AD patients, but also in mice, human neurons, and human brains. We have in fact compared the immunostaining profile of biosimilars of bapineuzumab, solanezumab, and crenezumab with NT4X in different mouse models and brains from patients with sporadic AD (Bouter et al., 2015) and found that NT4X has low plaque-binding potential in comparison with the other antibodies. We believe that this feature is therapeutically beneficial because it improves the antibody bioavailability for soluble Aβ in the CNS.